1. Field of the Invention
The present invention relates to a method of driving an ultrasonic transducer for use in measuring sound velocity in a liquid by transmitting and receiving an ultrasonic wave.
2. Description of the Related Art
In general, an ultrasonic transducer has a piezoelectric resonator including a pair of electrodes which sandwich a piezoelectric body, and is provided with a backing layer on the back surface of one of the electrodes of this piezoelectric resonator (for example, see Japanese Unexamined Patent Application Publication No. 2003-259490 (Patent Document 1)). When a drive signal is applied across the pair of electrodes, the piezoelectric resonator is excited to transmit an ultrasonic wave. On the other hand, when an ultrasonic wave is received, the piezoelectric resonator converts the vibration into an electrical signal, and outputs the electrical signal. In addition, the backing layer is provided in order to absorb and attenuate an ultrasonic wave emitted from the piezoelectric resonator to the back surface at the time of excitation.
When sound speeds in various liquids are measured using such an ultrasonic transducer, a pair of ultrasonic transducers are disposed at a predetermined distance from one another, and an ultrasonic wave is transmitted from one of the ultrasonic transducers. The other of the ultrasonic transducers receives the ultrasonic wave that has passed through a liquid. A measurement circuit measures the time required for transmission and receiving, and the sound speed in the liquid is calculated on the basis of the measured time and the distance between both of the ultrasonic transducers.
In this case, when the difference of the acoustic characteristic impedance (acoustic characteristic impedance) of the piezoelectric resonator and that of the backing layer disposed on the back surface thereof is large, the reflection of a sound wave occurs on the boundary surface of both layers to cause resonance in the piezoelectric body, and thus, a phenomenon in which vibration continues without converging in a short time, namely so-called ringing, occurs due to the resonance. When this ringing occurs, a ringing component is included in the signal of the received wave which causes increases in measurement errors and other problems such as lowering the time resolution. Accordingly, a known ultrasonic transducer has been proposed in which a setting is determined such that the acoustic characteristic impedance of the backing layer has substantially the same value as the acoustic characteristic impedance of the piezoelectric body constituting the piezoelectric resonator, and both of them are integrally bonded (for example, see Japanese Unexamined Patent Application Publication No. 2003-37896 (Patent Document 2)).
Incidentally, when applying a drive pulse to an ultrasonic transducer, the shape of the drive pulse is important. When a piezoelectric body is used in an ultrasonic transducer, the applied voltage pulse (=drive pulse) and the displacement have substantially the same waveform, and the sound pressure and the particle speed of the generated ultrasonic wave pulse has substantially the same waveform as the time differentiation of the applied voltage pulse. That is to say, when the drive pulse is a rectangular wave, the differentiation of a rise of a pulse becomes a peak that rises and falls, whereas a fall of a pulse becomes a valley that falls and rises. In short, when a drive pulse having a rectangular shape is applied, a sound wave is generated by the differentiation value of the pulse. Thus, for example, an ultrasonic waveform having two consecutive changes, a peak and a valley, is generated as a waveform of Td=350 nsec in FIG. 5. In this regard, when the element is driven using, for example, a transistor, the drive current is limited, and thus, the voltage across the terminals of the element is a triangular wave rather than a rectangular wave if the electrostatic capacity thereof is large. This causes a lowering of the sound pressure of the generated ultrasonic pulse, and thus, it is desirable to make the drive current as large as possible and to make the electrostatic capacity of the element as small as possible such that the voltage across the terminals is substantially a rectangular wave at implementation time. When the pulse width is wide, the peak and a valley are separated in time.
On the other hand, when the sound speed in various liquids as described above is measured, measurements are made of the time required from applying a drive pulse shown in FIG. 23A to an ultrasonic transducer to the receiving of the wave. At that time, for example, as shown in FIGS. 23B and 23C, if the receiving side measures T1, which is the time period required for the wave to reach the vicinity of the peak of the waveform or the valley, the measurement is likely to be affected by the gain of an amplifier, noises, and other interference, and thus, the measurement precision is likely to be deteriorated.
Accordingly, up to now, as shown in FIGS. 23D and 23E, the detection has been carried out on T2, which is the time of crossing the point of a zero amplitude during a fall from a peak toward the next valley, namely up to a zero-cross point.
In order to detect the zero-cross point of a received signal with high precision as described above, it is desirable that the gradient of the waveform at the zero-cross point is as sharp as possible. However, in a known technique, the waveform of the drive pulse to be applied to an ultrasonic transducer, particularly the pulse width, has not been fully examined. Thus, measurement errors increase. For example, the detection position is unclear when the zero-cross point is detected. Accordingly, problems, such as the decrease of time resolution, have occurred.
Also, when a setting is determined such that the acoustic characteristic impedance of the backing layer has substantially the same value as the acoustic characteristic impedance of the piezoelectric body defining the piezoelectric resonator as disclosed in Patent Document 2, it is possible to suppress the reflection on the boundary surface of both of the layers to a certain extent.
However, even in this case, the reflection of an ultrasonic wave occurs on the end surface of the open side, which is the opposite side of the backing layer to the boundary surface with the piezoelectric resonator, and thus, this reflection component is transmitted to the receiving side to cause measurement errors. Accordingly, it is necessary to prevent the influence of the reflection on the end surface of the open side of the backing layer.